The present invention relates to radar system simulations and more particularly relates to emulating the analog video signals and control signals produced by a search or beacon radar system.
The radar systems which can be emulated by the present invention are divided into two broad types: search radars and beacon radars. The following paragraphs are based on the type of radar systems which are employed in the civilian air traffic control environment. The terms defined are the common terms used in that environment.
For both radar types, rotating antennas are used and generally both radar antennas are mounted on the same rotating structure and aligned in the same direction. As the antennas rotate they periodically transmit electromagnetic pulses; these transmissions are known as radar sweeps. The rate of transmission is called the Pulse Repetition Frequency (PRF). Search and beacon radars can have different PRFs; however, they are synchronized to transmit at a known time difference when both radars are to transmit in the same sweep.
The analog output information from a radar system is referred to as radar video. The azimuth at which the antennas are pointing is provided to external equipment by the use of two signal lines. A north reference signal called the Azimuth Reference Pulse (ARP) is used to indicate when the radar is pointing north. One full rotation of the radar is divided into 4.096 Azimuth Change Pulses (ACPs). These pulses are transmitted serially during the antenna rotation to indicate azimuth. A complete antenna rotation is called an antenna scan, and begins with an Azimuth Reference Pulse (ARP) and ends just before the next ARP.
A search radar system is conceptually simple. As the antenna rotates through a complete scan it generates many sweeps. At the begining of each sweep an electromagnetic pulse of high energy and short duration is transmitted. A receiver detects the reflected electromagnetic energy (i.e. the radar return) and presents it to other equipment for processing or display. The time difference between transmission and return is used to determine the range of a target. The target azimuth is defined by the ARP and ACP signals.
A beacon radar system requires active participation by the target to enable the target to be detected. In this case an encoded set of pulses, called mode pairs, is transmitted by the beacon antenna. The transmission of these pulses is called an interrogation. A properly equipped aircraft has a device called a transponder which detects the pulse pair as valid and replies with response pulses. The response pulses contain either altitude information or an assigned aircraft identification number. The range and azimuth of the aircraft are determined in a manner similar to that used for search radar. The beacon radar transmits mode pair pulses separated by twenty-one microseconds to request altitude information. This is known as a Mode C interrogation. Similarly, the beacon radar transmits mode pair pulses separated by eight microseconds to request the aircraft identification number. This is known as a Mode 3/A interrogation. In each case the response from an aircraft is a serial bit train of 15 pulses, each pulse separated by 1.45 microseconds. The first and fifteenth pulses are always present and are called framing pulses. The inner thirteen pulses are data bits which carry the requested information, i.e. Mode C or Mode 3/A data. Occasionally an additional bit can be appended 4.35 microseconds after the last framing pulse. This bit is called the special position identifier bit and can be used to highlight a particular aircraft. Other interrogation modes exist but these modes are not normally used in civilian aviation.
Radar simulation systems have been proposed in which a list of aircraft are simulated in a digital computer and the proper response for each aircraft is presented by the computer to a hardware interface for output on a sweep to sweep basis. This type of system requires close coordination between the output hardware and the digital computer, which in turn requires a significant amount of complex high-speed hardware to present each aircraft's position at the proper time. Also, the requirement on the computer to service the output hardware each sweep presents a significant burden on the processing time of the computer. Another drawback of the prior art is that the aircraft must be processed in an azimuth arranged fashion. This requires that if any simulation of dynamic targets is attempted, sorting of aircraft must be continuously conducted to ensure the proper output sequencing. Most of the prior art is based on only the simulation of search radars.